Technical Field
The present invention generally relates to methods of forming silicon carbide fibers. In some specific embodiments, the present disclosure relates to methods of forming, curing, and using polysilazane resins.
Background Information
Polysilazane green fibers are precursors to silicon carbide (SiC) fibers. These green fibers must be cross-linked to make them infusible so that the fiber's dimensional integrity is maintained during subsequent pyrolysis steps. The current commercially viable crosslinking process of green fibers involves exposure of a package of polycarbosilane or polysilazane green fibers to high energy e-beam radiation. However, the high energy e-beam system is prohibitively expensive due to the large capital investment. Further, the currently practiced irradiation process takes several hours due to the requirement that the temperature of irradiated fibers not reach the melting point of polysilazane resin, wherein they would melt and become deformed. The required large e-beam dose, therefore, must be delivered at a slow rate and must take enough time to cool down before returning to the e-beam for another small dose, until such time as the package is effectively cross-linked.
Other prior art methods of making polysilazane green fibers infusible involve exposure of the fibers to moisture, which provides a cross-linked fiber with high level of oxygen. Some approaches involve the addition of a free-radical generator to a pre-ceramic polymer in an inert (that is, moisture- and air-free) atmosphere. Some specially formulated polysilazane green fibers may also be cross-linked by UV irradiation. Other methods involve exposing the green fibers to reactive and toxic gases such as ammonia, BCl3 or HSiCl3. Such processes present several environmental health and safety challenges and are expensive due to the toxic nature of the reagents.
Some prior art methods of forming silicon carbide structures are not conducive to forming silicon carbon fibers. For instance, in some cases, films are formed, rather than fibers. In these instances, melting of the polymeric structures is not only acceptable, but is often desired; this would be the case for films. Fibers, however, must be prevented from melting so that the desired shape is maintained. Thus, a need exists for a safe, inexpensive method for making silicon carbide fibers that do not become deformed during cure and production.